An acid anhydride is a nonmetal oxide which reacts with water to form an acidic solution. In organic chemistry, an acid anhydride is a functional group consisting of two acyl groups joined together by an oxygen atom.
Acid anhydride also refers to compounds containing the acid anhydride functional group. Acid anhydrides are named from the acids that created them. The 'acid' part of the name is replaced with 'anhydride'. For example, the acid anhydride formed from acetic acid would be acetic anhydride.